lol, not a stupid question, but it's not that easy. Well, ok, it's easy to do, but not easy to make look good.
First, the key filters in FCP are not very good. The Keylight filter in After Effects/Shake is great, and much more useful that the FCP keyers.
You need to shoot your subject against a green or blue screen. The lighting needs to be EVEN. You need good lighting, shadows will screw up the key, or at least make it harder to pull. You will benefit from a large area, with the green screen covering the wall and floor. Pull your subject as far from the screen as you can, so that their shadows do not drop on the screen and so that the screen color does not spill on the subject.
Use a key filter to pull the key. if your shooting in a 4:2:0 color space (DV) you won't be able to get a good key considering you've never done it before. There will be green haloing around the edges of the subject. You need a matte choker to eliminate that, and you should make the edges of your subject "blume" to make it look natural. Also, some spill supression will help since likely you will be shooting in a small space and spill is inevetable.
Then you need to have a shot to composite the key against. The two shots should be shot at the same level and angle so it looks natural. Put the shots ontop of each other in the timeline and vola, a quick and dirty chroma key.
The more time and research you put into it the better it'll be.
Check creativecow.net, they have a good keying tutorial that helps if you're shooting on DV, although it's done in AE, not sure what program you're using.
Luke
